How's the weather up there?: Science researchers look at snow and climate change on Mars

December 06, 2001

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High-resolution images of Mars' south pole show changes in pits, ridges, and mounds on the polar cap that suggest dramatic erosion of the cap's year-round frosty upper layers, a possible sign of global climate change, according to new research reported in the 7 December issue of the journal, Science.

A second paper in the same issue measures the depths of more ephemeral carbon dioxide "snow" that accumulates and evaporates seasonally on the martian surface. Reserving your interplanetary lift tickets is probably a bad idea, however: the martian snow is far from the powder craved by skiers, and is in fact much more dense and ice-like than loose snow or even compacted snowpacks on Earth, say the scientists.

Together, these two reports contribute new insights into Mars' carbon dioxide cycle that could help scientists better understand the planet's weather and climate, as well as provide information about the atmosphere and surface that could prove useful in future explorations.

The observations "herald a new era in the study of Mars in which older 'billiard ball' models of the planet are infused with hefty doses of reality," says David A. Paige of the University of California, Los Angeles, author of a related Perspective article discussing the two Science reports.

Pictures of pits and other features on Mars' south pole, captured by the Mars Orbiter Camera (MOC) onboard the orbiting Mars Global Surveyor, confirm the presence of a year-round reservoir of solid carbon dioxide on the martian surface, according to the authors of the first Science report. They suggest that gas exchange between the atmosphere and this reservoir could have significant effects on the planet's long-term climate and climate stability.

"The fact that we see the reservoir means that not all the carbon dioxide that can be in the atmosphere is in the atmosphere today. This means that the climate is really dynamic and changing with time," says Michael C. Malin of Malin Space Science Systems.

The size of the reservoir, still unknown, could also affect phenomena like the presence of liquid water on Mars' surface. If the reservoir is large, for example, the atmospheric pressure change brought about by its evaporation could create conditions that "permit liquid water to persist at or near the surface for as long as the pressure is elevated," according to Malin.

In 1999, MOC made its first observations of irregular to circular-shaped pits, intervening ridges, and isolated mounds carved into apparent layers of frost at the southern pole. To search for changes in these structures over time, Malin and colleagues used MOC to re-image specific features in 2001.

Comparisons between the two sets of images suggest that many of these pits are growing, simultaneously shrinking the ridges between them, and that many small features have disappeared altogether. Calculations made by the researchers indicate that 25 to 50 percent of the features measured have been eroded by one to three meters during the course of the past martian year.

The amount of erosion seen in these structures suggests that the frost layers are composed of moderately dense solid carbon dioxide, rather than water ice, say the Science authors.

With only a year's data available, it's too soon to know when this erosion began, how long it will continue, or where the evaporated carbon dioxide is going, say the researchers. Based on the observed erosion rates, however, the pits could have been created in a Mars' decade and may erode away completely within one to two decades.

"We know that the pits we see at the surface today are not very old, and that they will not last very long. These layers of carbon dioxide are very ephemeral on a geological timescale," says Malin.

Along with the year-round reservoir, solid carbon dioxide also takes a more seasonal form on the martian surface. Over the course of a year, Mars exchanges up to a third of the carbon dioxide in its atmosphere with its surface, depositing layers of dry ice "snow" at the northern and southern polar regions during each hemisphere's autumn and winter and evaporating the deposits during the spring and summer.

With the help of another instrument onboard the Mars Global Surveyor, David E. Smith, Maria T. Zuber and Gregory A. Neumann of the NASA/Goddard Space Flight Center and the Massachusetts Institute of Technology measured elevation changes on the martian surface that correlate with this seasonal cycle of snow accumulation and evaporation.

"We have measured when and where the carbon dioxide resides during the winter over the entire surface of Mars, which will allow more accurate models of martian weather to be developed," says Zuber.

The data should improve models of the martian atmosphere as well, important considerations in aerobraking spacecraft and choosing landing targets, according to Zuber.

The Surveyor's Mars Orbiter Laser Altimeter (MOLA) measures the elevation of the martian landscape by beaming a pulse of laser light down to the surface and calculating the amount of time it takes for the pulse to bounce back to the Surveyor. The researchers analyzed over 400 million MOLA measurements, accurately tracking elevation changes related to snow depth to within ten centimeters.

The biggest elevation changes--representing differences in snow depth across the seasons--were observed at high latitudes above 80 degrees, close to the poles. The bulk of snow accumulation and evaporation appears to take place at lower latitudes, however, below 75 degrees in the north and 73 degrees in the south.

The Science researchers also documented, for the first time, "off-season" snow accumulation and evaporation on Mars. In one particular case, they observed unusual autumn evaporation in the northern hemisphere, possibly related to regional dust storms.

Zuber notes that, as on Earth, martian weather may act in unexpected ways.

"What we don't yet understand is what causes deviations from the expected trends. Does carbon dioxide condense in shadowed areas? Or are there local intense storms that the current models of the Martian atmosphere don't predict?"

Zuber and colleagues also measured the tiny variations in Mars' gravity field that occur as a result of the seasonal redistribution of snow between the poles. Using this data along with the elevation observations, the researchers calculated the density of the carbon dioxide snow, determining that it is much more dense than snow on Earth.
The other members of the Malin team include Michael C. Caplinger and Scott D. Davis of Malin Space Science Systems. This research was funded by NASA through the Jet Propulsion Laboratory (JPL).

The Smith et al. research was supported in part by the NASA Mars Exploration Program.

American Association for the Advancement of Science

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